The present invention relates to a switching circuit employing a switching element. Also, the invention relates to a DC-DC converter unit for boosting or lowering voltage from a direct current power source, a power source enhancing power factor of an alternating current power source, a DC-AC converter unit for converting current from a direct current source into a desired frequency of alternating current, and an AC-DC converter unit for converting current from an alternating current source into a direct current, employing the switching circuit of the invention.
As a conventional switching system for reducing switching loss in a switching element, a resonance type system, in which the switching element, a reactor and a capacitor are combined, has been disclosed in a Japanese Journal "DENSHI GIJUTSU" (or "Electronic Technologies"), Nikkan Kogyo Shinbun K.K., special edition, Vol. 132, No. Mar. 3, 1990, pp. 9-19. In that publication, an apparatus such as shown in FIG. 2 herein has been disclosed as a resonance type power source unit for a DC-DC converter unit for boosting current from a direct current power source.
In FIG. 2, the reference numeral 1 denotes a direct current source, 2 denotes a direct current reactor, 3 denotes a resonance reactor, 4 denotes a switching element, 5 denotes a resonance capacitor, 6 and 7 denote diodes, 8 denotes a capacitor and 9 denotes a load.
With the above-mentioned circuit construction, when the primary switching element 4 is turned OFF, a resonance current IL flowing through the resonance reactor 3 will be sinusoidal as illustrated in FIG. 3. Then, the voltage Vbe on the base of the primary switching element 4 is switched OFF during a period TB in which a reverse current is flowing through the diode 7. This makes it possible to turn OFF the switching element with zero current for suppressing switching loss.